Clinical and necropsy observations are described in a man in whom 2 sheep carotid arterial heterografts were inserted as aortocoronary bypass conduits 30 months before death, and in whom a canine saphenous vein heterograft was inserted several hours before death. Examination of previous reports describing heterografts for vascular reconstruction or bypass in both man and experimental animals and observations in our patients suggest that heterografts are unsatisfactory as aortocoronary bypass conduits.